The invention that is the subject of this application was invented by the following:
Conventional processes for the separation and recovery of products in a fluid catalytic cracking unit (also referred to as a xe2x80x9ccat cracker,xe2x80x9d xe2x80x9ccat cracking,xe2x80x9d or xe2x80x9cFCCxe2x80x9d unit) of a petroleum refinery do not provide for efficient recovery of the increased quantities of light olefinic products that may be desirably produced with advances in cracking catalyst technology and cat cracker design. The present invention is a new process for the improved separation and recovery of products, especially ethylene and propylene, produced in the cracking reaction of a cat cracker. More specifically, the present invention is a new process for separation and recovery of products from a cat cracker such as the Deep Catalytic Cracking unit described in more detail herein that utilizes recent advances in technology for increased ethylene and propylene production.
In the cracking reaction of conventional fluid catalytic cracking units of petroleum refineries, a hydrocarbon feedstock may be catalytically converted into a variety of products commonly known as slurry oil, heavy cycle oil, light cycle oil, naphtha, and various components lighter than naphtha. The term xe2x80x9cnaphthaxe2x80x9d as used herein means a process stream that contains predominantly five carbon and heavier chemical components with an end point of approximately 430xc2x0 F. The term xe2x80x9cnaphthaxe2x80x9d as used herein may include a debutanized stream of cracked hydrocarbons that may be processed and used, for example, as a gasoline blending stock. Of course, the particular products from any particular cat cracker unit depend on a variety of factors including the design and needs of the petroleum refinery. The products that include naphtha and lighter components are separated into various product streams in the section of the refinery commonly referred to as the xe2x80x9cgas plant,xe2x80x9d xe2x80x9cgas concentration plant,xe2x80x9d xe2x80x9cgas recovery unit,xe2x80x9d or xe2x80x9cunsaturated gas plantxe2x80x9d of the cat cracker unit. These terms are commonly used to refer to that portion of a cat cracking unit that includes the wet gas compressor and equipment downstream of the compressor. The products from the gas plant vary depending on the particular refinery design but commonly include naphtha (as defined herein), C4s (butylenes and butanes), propane, propylene, and a stream (commonly referred to as xe2x80x9cfuel gasxe2x80x9d) that contains ethane and lighter components (C2 and lighter).
Importantly, conventional cat cracker gas plant processes separate, at the front end of the gas plant, an xe2x80x9cethane and lighterxe2x80x9d stream from a xe2x80x9cpropylene and heavierxe2x80x9d stream. Such conventional cat cracker gas plant processes that include a deethanization step at the front end of the gas plant are described in various treatises. See Nelson""s Petroleum Refinery Engineering (McGraw Hill 1949, pp. 759-810); Meyer""s Handbook of Petroleum Refining Processes (Second edition 1997, pp. 3.1-3.112); Petroleum Refinery Distillation by Watkins, Second Edition. The deethanization step at the front end of the cat cracker gas plant, and the other features of the conventional cat cracker gas plant as described herein, do not provide for efficient separation of the light olefinic components (such as ethylene and propylene) of the cat cracker reaction products.
In the conventional cat cracker gas plant, the ethane (C2) and lighter stream may be routed to the refinery fuel gas system or further processed to separate ethane and ethylene from the methane and lighter components. The propylene and heavier stream from the front end of a conventional gas plant is typically further processed to separate a stream that is a naphtha (predominantly C5+) fraction from the C4 and lighter fraction, which may also be further processed to separate the C3s (propane and propylene) and C4s or processed directly in an alkylation unit.
The product yields from a conventional cat cracker unit vary depending on a wide range of design and operating parameters, such as feedstock quality, the amount of regenerated catalyst supplied to the riser reactor per volume or mass unit of feed, the temperature at which the cracking reaction takes place, the residence time of the feed in the riser reactor, and the like. The conventional fluid catalytic cracking unit may process one or more feedstocks. Typical feedstocks may include atmospheric gas oils, heavier feedstocks from vacuum distillation units, and streams from other units such as cokers, visbreakers, hydrotreaters, and hydrocrackers. The design criteria for conventional fluid catalytic cracking units depend on the feedstock quality, the amount of coke formed on the cracking catalyst during the reaction, the level of contaminants in the feedstock (such as metal contaminants that deactivate the cracking catalyst) and the like.
New catalysts and cat cracker designs have been developed recently in an effort to respond to increased demand for olefinic products from the fluid catalytic cracking unit of a refinery. These recent developments for increased olefin production include changes to the zeolite catalysts typically used in cat crackers and changes in plant process equipment design. For example, the xe2x80x9cDeep Catalytic Crackingxe2x80x9d (or xe2x80x9cDCCxe2x80x9d) process yields increased proportions of olefinic compounds in comparison to conventional fluid catalytic crackers. As used herein, the term Deep Catalytic Cracking means the process described in U.S. Pat. Nos. 4,980,053 and 5,326,465, the disclosures of which are incorporated herein by reference, and a process that utilizes the catalyst disclosed in U.S. Pat. Nos. 5,232,675, 5,358,918, and 5,380,690, the disclosures of which are incorporated herein by reference. Also see the Handbook of Petroleum Refining Process, second edition, (1997) edited by Robert A. Meyers and published by McGraw-Hill, Chapter 3.5 (pp. 3.101-3.112) entitled xe2x80x9cDeep Catalytic Cracking, the New Lights Olefin Generator,xe2x80x9d the disclosures of which are incorporated herein by reference. DCC is a fluidized catalytic process for selectively cracking a variety of feedstocks to light olefins. Unlike a steam cracker, the predominant products from a DCC unit are propylenes and butylenes, the direct result of catalytic cracking rather than free radical thermal reactions. The DCC unit may be operated in two distinct modes: Maximum Propylene (Type I) or Maximum Iso-Olefin (Type II). Each mode of operation employs a unique catalyst as well as reaction conditions. DCC reaction products are light olefins (such as ethylene and propylene), high octane gasoline, light cycle oil, dry gas and coke. A small amount of slurry oil is produced.
DCC Maximum Propylene operation (Type I) employs both riser and bed cracking at severe reactor conditions. DCC Maximum Iso-Olefin (Type II) operation utilizes riser cracking, like a modem FCC unit, at slightly milder conditions than a Type I operation.
Each mode of DCC operation employs a unique catalyst as well as reaction conditions. DCC reaction products are light olefins, high octane gasoline, light cycle oil, dry gas and coke. As used herein, dry gas means a stream of hydrogen and methane. A small amount of slurry oil is produced.
Innovations in the areas of catalyst development, process variable selection and anticoking techniques enables the DCC unit to produce significantly more olefins than a conventional FCC unit. Table 1 compares pilot plant product yields for a Deep Catalytic Cracking Unit and a conventional cat cracker (maximum gasoline mode) and illustrates the increased light olefin production from a DCC Unit.
As used in Table 1, xe2x80x9cC2 Minusxe2x80x9d refers to ethane and lighter components; xe2x80x9cC3 and C4xe2x80x9d refer to three and four carbon compounds respectively; xe2x80x9cNaphthaxe2x80x9d has the meaning previously described herein; xe2x80x9cLCOxe2x80x9d refers to light cycle oil; xe2x80x9cDOxe2x80x9d refers to decant oil, also known as slurry oil; and xe2x80x9ccokexe2x80x9d refers to carbonaceous deposits on catalyst resulting from the cracking reaction. The Olefin Yield section of Table 1 shows weight percentages of specific olefin compounds.
Historically, the light gases such as ethylene and propylene separated in the cat cracker gas plant were of lesser value and concern than the gasoline product. However, because olefins are now a significant product from a cat cracker and an important source of revenue, there is currently a need to increase the production and recovery of these olefinic products, such as propylene and ethylene.
As noted in Table 1, the Deep Catalytic Cracking Unit produces more ethylene and propylene than a conventional cat cracker. Whereas a conventional cat cracker reactor typically produces up to approximately 5 weight percent or slightly higher (based on reactor fresh feed mass rate) propylene, proprietary catalytic cracking units that are specifically designed to produce more propylene (such as the Deep Catalytic Cracking unit) can produce up to approximately 20 weight percent (based on fresh feed mass rate) propylene in the reactor.
Existing catalytic cracking units that were not originally designed and constructed for increased olefin production may be modified or xe2x80x9crevampedxe2x80x9d to incorporate new technology that increases the production of olefinic compounds such as ethylene and propylene. The addition of catalytic cracking catalyst specifically formulated for increased ethylene and/or propylene yields in the catalytic cracking reaction will result in the need for a new gas plant process that efficiently accommodates the increased light olefin production. In addition to catalyst changes, the process equipment of an existing cat cracker may be revised to provide increased yields of light olefins. For example, changes to the riser/reactor and related equipment may provide increased olefin production.
Propylene recovery (the portion of propylene in the gas plant feed recovered as propylene produce in conventional gas plants ranges from 80-95 percent (with the remainder of the propylene being lost to other streams). Some refiners attempt to reduce the amount of light olefins lost to fuel gas streams with a xe2x80x9cstand-alonexe2x80x9d recovery train to separate C2 and C3 olefins from the lighter gases, such as the C2 and lighter gases from the deethanization step at the front end of the conventional cat cracker gas plant. However, ethylene and propylene recovery via such a stand alone system dedicated to processing the light gases destined for the fuel gas system results in redundancy of expensive processing equipment. Moreover, in gas plant processes with a front-end deethanizing step accomplished in an absorber tower followed by a stand-alone recovery train for the C2 and lighter stream, the C4 content of the propane recovered from the C2 and lighter stream may fluctuate due to swings in the C4 content of the absorber overhead.
Therefore, in light of new processes with increased olefin yields, such as the Deep Catalytic Cracking process, and the increased light olefin yields that may be accomplished through changes to existing cat cracker units, there exists a need for a new gas plant process that provides efficient recovery of ethylene and propylene from such processes and units.
The invention relates to a new process for more efficient separation and recovery of light olefins (such as ethylene and propylene) in a cat cracker gas plant and is particularly advantageous when used in a DCC unit gas plant. Specifically, as but one aspect of the new process, the new cat cracker gas plant process separates, at the front end of the gas plant, a xe2x80x9cpropane and lighterxe2x80x9d stream from a xe2x80x9cC4 and heavierxe2x80x9d stream. This new process that includes a xe2x80x9cdepropanizationxe2x80x9d step at the front end of the cat cracker gas plant (as opposed to the front end deethanization step of the prior art) results in a stream that contains predominantly C4 and heavier components and a stream that contains predominantly C3 and lighter components. The resulting stream that contains propane and lighter components (including but not limited to substantially all of the ethylene and propylene formed in the cat cracking reaction and fed to the gas plant) is further processed via compression, chilling (cooling), and distillation to maximize olefins recovery. The new process is particularly useful for efficiently separating and recovering olefins from a Deep Catalytic Cracking unit that produces increased levels of olefins via advances in cracking catalyst technology and/or design features.
The new process described herein achieves, among other advantages, increased light olefin recovery efficiency while eliminating the installation and operating costs for absorber towers and associated equipment employed in conventional cat cracker gas plants for the separation of propylene and heavier components from a stream containing ethane and lighter component. In this new process, the need for one or more absorber towers to remove propylene from a C2 and lighter stream, and the expensive processing of the lean/rich oil streams associated therewith, are eliminated. The elimination of the need for a lean oil stream reduces energy costs and, in conventional cat cracker units wherein the rich oil used for absorption of C3 and heavier components is recycled to the distillation tower that processes effluent directly from the cat cracker reactor (known to those skilled in the art as the Main Fractionator), reduces the amount of material that must be processed in the Main Fractionator and thereby may reduce the height of the Main Fractionator or allow for additional pumparound capabilities for the Main Fractionator.
It has been discovered that the need for a contaminant removal system that removes contaminants such as hydrogen sulfide and carbon dioxide on the overhead of the Debutanizer tower of the conventional cat cracker gas plant (which is described in detail below) is eliminated because significant quantities of such contaminants do not enter the gas plant Debutanizer tower in the new process; rather, these contaminants appear in the C3 and lighter fraction separated at the front end of the new cat cracker gas plant process. The C4+ (butylene and heavier) stream separated at the front end of the new gas plant process contains so little contaminants (such as hydrogen sulfide and carbon dioxide) that no contaminant removal (such as H2S or CO2 removal) is required on the C4+ stream.
The new process will provide better control over the specification for the C4+ content of a propane product stream because fluctuations in composition associated with the operation of a front end gas plant deethanizing absorber of the prior art are eliminated.
In this new process, the gas plant Debutanizer tower may be smaller than that of a conventional cat cracker gas plant because the Debutanizer feed is low in C3 content. Moreover, the operating pressure and bottoms temperatures are lower in the Debutanizer tower of the new process when compared to conventional cat cracker gas plant Debutanizer towers. A lower Debutanizer tower bottoms temperature is desirable because fouling of the Debutanizer tower and associated equipment is accelerated at higher Debutanizer tower bottoms temperatures.
The cooling requirements of the new gas plant process are such that the need for an ethylene refrigeration system associated with gas plants of the prior art is eliminated, thus reducing installation costs and operating costs. Of course, if necessary or desirable due to a particular plant""s specifications or feedstocks, an ethylene refrigeration system or other refrigeration system, such as a mixed ethylene/propylene refrigeration system, may also be used with the new process.
The new process invention for recovering olefins from a mixture of cracked hydrocarbons from a fluid catalytic cracker comprises the steps of: (a) providing a mixture of cracked hydrocarbons including methane, ethylene, ethane, propylene, propane, butylene, butane and heavier hydrocarbons such as naphtha produced in a fluid catalytic cracker; (b) separating said mixture into (i) a first stream comprising substantially all of said ethane, ethylene, and methane and a major portion of said propane and propylene and (ii) a second stream comprising a portion of said butylene and butane, and a major portion of said heavier hydrocarbons; and (c) processing said first stream to recover the ethylene and propylene therefrom, and the details of such process described herein.